U.S. patent number 6,217,422 [Application Number 09/233,886] was granted by the patent office on 2001-04-17 for light energy cleaning of polishing pads.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Daniel L. Franca, Raymond M. Khoury, Jose M. Ocasio, Uldis A. Ziemins.
United States Patent |
6,217,422 |
Franca , et al. |
April 17, 2001 |
Light energy cleaning of polishing pads
Abstract
A method and apparatus for cleaning polishing debris from the
surface of a polishing pad by wetting the surface with a liquid and
irradiating the wetted surface with a beam of light. The light beam
has sufficient intensity at the polishing surface of the pad to
vaporize at least a portion of the liquid such that the vaporized
liquid causes at least a portion of the debris to be expelled from
the polishing surface of the pad.
Inventors: |
Franca; Daniel L.
(Poughkeepsie, NY), Khoury; Raymond M. (Wappinger Falls,
NY), Ocasio; Jose M. (Maybrook, NY), Ziemins; Uldis
A. (Poughkeepsie, NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
22879074 |
Appl.
No.: |
09/233,886 |
Filed: |
January 20, 1999 |
Current U.S.
Class: |
451/56; 451/444;
451/60 |
Current CPC
Class: |
B24B
37/04 (20130101); B24B 53/017 (20130101) |
Current International
Class: |
B24B
53/007 (20060101); B24B 37/04 (20060101); B24B
001/00 () |
Field of
Search: |
;451/56,59,60,72,443,444,41 ;134/1.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Eley; Timothy V.
Assistant Examiner: Nguyen; Dung Van
Attorney, Agent or Firm: Pollock, Vande Sande &
Amernick
Claims
What is claimed is:
1. A method for cleaning a polishing surface of a polishing pad
containing polishing debris embedded in features of said polishing
surface, said method comprising steps of:
supporting the pad to be cleaned on a supporting surface with the
polishing surface exposed;
wetting features of the polishing surface of the pad with a liquid;
and
irradiating the wetted surface features of the pad with a beam of
light of sufficient intensity to vaporize a sufficient portion of
said liquid for the vaporized liquid to cause at least a portion of
said embedded debris to be dislodged and expelled from said surface
features without removing said features of the polishing surface of
the pad.
2. A method according to claim 1 further comprising a step of
causing relative movement between said pad surface and said light
beam.
3. A method according to claim 2, wherein said light beam moves
relative to said pad surface.
4. A cleaning method according to claim 3, wherein movement of said
light beam is caused by movement of a mirror reflecting said light
beam towards said pad surface.
5. A cleaning method according to claim 4, wherein said mirror is
mounted in a housing arranged for movement in a lateral direction
relative to said reflected light beam, and said housing is caused
to reciprocate in said lateral direction.
6. A cleaning method according to claim 4, wherein said mirror is
mounted in a housing having a fixed position relative to said pad
surface, and wherein said relative movement of the light beam is
caused by relative motion between said mirror and said housing.
7. A cleaning method according to claim 2, wherein said pad surface
moves relative to said light beam and said liquid is water.
8. A cleaning method according to claim 7, wherein said supporting
surface is mounted for rotation so that at least a portion of said
pad surface rotates past an intersection between said light beam
and said pad surface.
9. A method according to claim 1, wherein said light beam is a
focused light beam provided by a laser, and the energy of said
light beam at the surface of said pad is a minimum of 250
mj/cm.sup.2.
10. A cleaning method according to claim 1, wherein said light beam
is telecentric.
11. A cleaning method according to claim 1, wherein said
irradiating step comprises reflecting a source beam of light
towards said pad surface with a first mirror to provide a first
beam of light of sufficient intensity to vaporize at least a
portion of said liquid, and reflecting said source beam of light
toward said pad surface with a second mirror to provide a second
beam of light of sufficient intensity to vaporize at least a
portion of said liquid; and wherein said second mirror is movable
between a first position out of a path of said source beam to said
first mirror, and a second position in said path to intercept said
source beam before it reaches said first mirror.
12. A method according to claim 1, wherein said surface features
are waves, holes, creases, ridges, slits, depressions, protrusions,
gaps, or pores.
13. An apparatus for cleaning a polishing surface of a polishing
pad containing polishing debris embedded in features of said
polishing surface, said apparatus comprising:
a support having a surface for supporting the pad with the
polishing surface exposed;
a nozzle arranged to wet features of the polishing surface of the
pad with a liquid spray; and
an optical system arranged to irradiate the wetted surface features
of the pad with a beam of light of sufficient intensity to vaporize
a sufficient portion of said liquid for the vaporized liquid to
cause at least a portion of said embedded debris to be dislodged
and expelled from said surface features without removing said
features of the polishing surface of the pad.
14. An apparatus according to claim 13 further comprising a means
for causing relative movement between said pad surface and said
light beam.
15. A cleaning apparatus according to claim 14, wherein said
movement means comprises a drive mechanism arranged to cause said
pad surface to move relative to said light beam, and wherein said
liquid is water.
16. A cleaning apparatus according to claim 15, wherein said
supporting surface is mounted for rotation and said drive mechanism
rotates said supporting surface so that at least a portion of said
pad surface rotates past an intersection between said light beam
and said pad surface.
17. An apparatus according to claim 13, comprising a means for
causing said light beam to move relative to said pad surface.
18. A cleaning apparatus according to claim 17, wherein said
movement means comprises a mirror for reflecting said light beam
towards said pad surface, and a drive mechanism for moving a said
mirror to cause said light beam to move relative to said pad
surface.
19. A cleaning apparatus according to claim 18, wherein said mirror
is mounted in a housing arranged for movement in a lateral
direction relative to a direction in which said light beam is
reflected by said mirror, and wherein said drive mechanism is
arranged to caused said housing to reciprocate in said lateral
direction.
20. A cleaning apparatus according to claim 18, wherein said mirror
is mounted in a housing having a fixed position relative to said
pad surface, and wherein said drive mechanism is arranged to caused
relative motion between said mirror and said housing such that said
light beam is caused to move across said pad surface.
21. A cleaning apparatus according to claim 13, wherein said light
beam is a focused light beam provided by a laser and the energy of
said light beam at the surface of said pad is a minimum of 250
mj/cm.sup.2.
22. A cleaning apparatus according to claim 13, wherein said light
beam is telecentric.
23. A cleaning apparatus according to claim 13, wherein said
optical system comprises a first mirror arranged to reflect a
source beam of light towards said pad surface to provide a first
beam of light of sufficient intensity to vaporize at least a
portion of said liquid, a second mirror arranged to reflect said
source beam of light towards said pad surface to provide a second
beam of light of sufficient intensity to vaporize at least a
portion of said liquid, and a reciprocating shuttle arranged to
move said second mirror between a first position out of a path of
said source beam to said first mirror and a second position in said
path to intercept said source beam before it reaches said first
mirror.
24. An apparatus according to claim 13, wherein said surface
features are waves, holes, creases, ridges, slits, depressions,
protrusions, gaps, or pores.
25. An apparatus for cleaning a polishing surface of a polishing
pad containing polishing debris in features of said polishing
surface, said apparatus comprising:
a support having a surface for supporting the pad with the
polishing surface exposed;
a liquid source arranged to wet the polishing surface of the pad
with a liquid; and
an optical system arranged to irradiate the wetted surface of the
pad with a beam of light of sufficient intensity to vaporize at
least a portion of said liquid such that the vaporized liquid
causes at least a portion of said debris to be expelled from said
features without removing said features of the polishing surface of
the pad;
said optical system comprising a first mirror arranged to reflect a
source beam of light towards said pad surface to provide a first
beam of light of sufficient intensity to vaporize at least a
portion of said liquid, a second mirror arranged to reflect said
source beam of light towards said pad surface to provide a second
beam of light of sufficient intensity to vaporize at least a
portion of said liquid, and a reciprocating shuttle arranged to
move said second mirror between a first position out of a path of
said source beam to said first mirror and a second position in said
path to intercept said source beam before it reaches said first
mirror.
Description
FIELD OF THE INVENTION
The present invention relates to the field of cleaning polishing
pads, and more specifically to cleaning polishing pads used in
chemical-mechanical planarization (CMP) of semiconductor
substrates.
BACKGROUND OF THE INVENTION
Chemical-mechanical polishing has been used for many years as a
technique for polishing optical lenses and semiconductor wafers.
More recently, chemical-mechanical polishing has been developed as
a means for planarizing intermetal dielectric layers of silicon
dioxide and for removing portions of conductive layers within
integrated circuit devices as they are fabricated on various
substrates. For example, a silicon dioxide layer may cover a metal
interconnect conformably such that the upper surface of the silicon
dioxide layer is characterized by a series of non-planar steps
corresponding in height and width to the underlying metal
interconnects.
The step height variations in the upper surface of the intermetal
dielectric layer have several undesirable characteristics. Such
non-planar dielectric surfaces may interfere with the optical
resolution of subsequent photolithographic processing steps, making
it extremely difficult to print high resolution lines. Another
problem involves the step created in the coverage of a second metal
layer over the intermetal dielectric layer. If the step height is
relatively large, the metal coverage may be incomplete such that
open circuits may be formed in the second metal layer.
To combat these problems, various techniques have been developed to
planarize the upper surface of the intermetal dielectric layer. One
such approach is to employ abrasive polishing to remove the
protruding steps along the upper surface of the dielectric layer.
According to this method, a silicon substrate wafer is mounted face
down beneath a carrier and pressed between the carrier and a table
or platen covered with a flat pad continuously covered with a
slurried abrasive material (in FIG. 1, a prior art abrasive disk
and holder are shown in the same position as such a wafer and
carrier).
Means are also provided for depositing the abrasive slurry on the
upper surface of the pad and for forcibly pressing the substrate
wafer against the polishing pad, such that relative movement of the
platen and the substrate wafer relative to each other in the
presence of the slurry results in planarization of the contacted
face of the wafer. Both the wafer and the table may be rotated
relative to each other to rub away the protruding steps. This
abrasive polishing process is continued until the upper surface of
the dielectric layer is substantially flat.
Conventional polishing pads may be made of a uniform material such
as polyurethane or may be formed from multilayer laminations having
non-uniform physical properties throughout the thickness of the
pad. Polyurethane polishing pads are typically formed by reacting
the reagents that form polyurethane within a cylindrical container.
After forming, a cylindrically shaped piece of polyurethane is cut
into slices that are subsequently used as the polishing pad. A
typical laminated pad may have a plurality of layers, such as a
spongy and resilient microporous polyurethane layer laminated onto
a firm but resilient supporting layer comprising a porous polyester
felt with a polyurethane binder. Polishing pads typically may have
pores that have a size of about 100-200 microns.
Conventional polishing pads typically may also have microtextured
surfaces resulting from a microscopic bulk texture of the pad
resulting from factors intrinsic to the manufacturing process. Some
of the factors which influence the microscopic bulk texture of a
conventional pad are the nature or texture of the work surface,
such as waves, holes, creases, ridges, slits, depressions,
protrusions, gaps or other spaces, and the size, shape and
distribution frequency or spacing of such features. Since polishing
does not normally occur across the entire pad surface, the
microtexture of the pad or any macrotextures made by surface
machining, may only be formed into the portion of the pad over
which polishing is to take place.
During the polishing process, the material removed from the wafer
surface and the abrasive, such as silica, in the slurry become
embedded in the pores and other free spaces within the microscopic
and macroscopic bulk texture of the polishing pad at and near its
surface. One factor in achieving and maintaining a high and stable
polishing rate is providing and maintaining the pad in a clean
condition. Pad cleaning or reconditioning is a technique whereby
the pad surface is returned to a proper state for subsequent
polishing work. The purpose of the cleaning method selected is to
remove the contaminating particles of wafer, abrasive or other
debris from the free spaces or interstices at or slightly below the
pad surface. Prior art cleaning techniques include diamond grit
conditioners that grind away the contaminated layer of the pad,
thereby removing the contaminants along with a portion of the pad
material itself.
FIG. 1 is an illustration of using a diamond grit conditioner to
clean a polishing pad 10 attached to a platen 14. Above the
polishing pad 10 is a holder 12 for carrying a diamond grit disk 11
and pressing it against the contaminated face 13 of the polishing
pad 10. During reconditioning, the carrier 12 is rotated by drive
shaft 15 and the platen 14 is rotated by drive shaft 16. The
carrier 12 and the platen 14 may rotate either clockwise or
counterclockwise, but typically the carrier 12 rotates in the same
direction as the platen 14. While the carrier and platen are
rotated, the carrier 12 may be oscillated back and forth across the
polishing pad as indicated by the arrow O. In this direct contact
reconditioning technique, the abrasive disk 11 and the carrier 12
are essentially substituted for the respective wafer and its
carrier mentioned in the prior art planarization technique also
described above.
Obviously, such direct contact processes significantly reduce the
life of the polishing pad. To avoid such degradation of the pad,
other prior art techniques include using a water knife or an air
knife in an attempt to blow away the contaminants. However, the
water or air velocities required for effectiveness are such that
substantial portions of the pad adjacent to its surface may also be
removed or damaged.
It has also been determined that ineffective reconditioning of the
polishing pad has a direct correlation to the amount of scratching
caused to a wafer surface by polishing with the poorly
reconditioned pad. This in turn causes significant losses in the
yield of semiconductor wafers. Thus, the consistency and quality of
polishing pad reconditioning by prior processes has presented two
problems. The first of these is the reduction in polishing pad life
due to the destructive nature of the reconditioning techniques
previously available. Secondly, the poor quality of polishing pad
reconditioning provided by prior art processes has resulted in a
relatively high loss of product yield due to wafer damage caused by
the reconditioning technique. Therefore, there is a need for a
polishing pad reconditioning process of higher quality and
consistency than provided by prior art processes.
SUMMARY OF INVENTION
The present invention provides a cleaning method for achieving a
high quality and consistency of polishing pad reconditioning by the
use of a laser or other collimated light source. A beam of high
intensity light from the light source is projected onto the surface
of the pad while the pad is rotated on a supporting surface, which
may be the same table or platen on which the pad is mounted for
polishing the surface of semiconductor wafers. The projected beam
may be moved radially relative to the rotating pad by moving the
optical system relative to the rotational axis of the platen or by
using a scanning beam delivery system. Regardless of how the light
beam is moved relative to the pad surface, the light beam is
concentrated so as to maintain a relatively uniform and high energy
density over a relatively small area of the pad surface while the
pad rotates past this irradiated area.
The light energy is applied to the pad surface in the presence of
water or another liquid to generate steam or vapor to assist in the
cleaning process. The steam or vapor generated acts to dislodge
particulates embedded in the thin contaminated layer of the pad
adjacent to its surface. The resultant steam or vapor cleaning
dislodges and removes particulates that would not be removed by
prior art techniques. A gas nozzle or knife may also be used with
the light source to provide an air or other gas assist mechanism to
further enhance contaminant removal and pad reconditioning. The
cleaning is preferably done by application of the light energy at a
shallow incidence angle relative to the plane of the pad
surface.
The present invention has a number of advantages over prior art
processes. Since it is a non-contact process, it results in
substantially less deterioration of or damage to the polishing pad,
particularly when compared with abrasive conditioners that remove
the contaminating layer adjacent to the pad surface. Also, abrasive
conditioners wear relatively rapidly and have unpredictable life
times, whereas the light energy source of the present invention may
be monitored and controlled to assure constant and repeatable
process parameters. Also, the wearing of abrasive conditioners
provide an additional source of polishing pad contamination, which
is avoided by the present invention.
The present invention can be incorporated in new or existing CMP
polishing devices with minimal modification of existing designs so
as to apply the light energy to conventional polishing pads to
clean the pads while they remain in place after periodic intervals
of polishing operations. The light source and optics providing the
light energy can easily be scanned to provide complete coverage of
the portions of the pad over which polishing takes place, and such
scanning can also be used to displace dislodged contaminants over
and beyond the edge of the platen on which the pad is carried.
The use of a laser, a collimated beam or other light source also
provides the capability of adjusting the grazing angle of the light
beam relative to the plane of the pad surface so that the
illuminated area can be used as a knife edge to selectively remove
coated and embedded contaminants instead of pad material. Thus, a
shallow grazing angle provides a shallow penetration below the
polishing pad surface and can be limited to the shallow depth of a
contaminated layer. The energy density at the work surface of the
pad also may be precisely controlled by adjusting one of the
various degrees of freedom provided by a light source, such as
frequency, energy output, focus, duty cycle and like.
BRIEF DESCRIPTION OF THE DRAWINGS
The structure, operation and advantages of the invention may be
better understood from the following detailed description of the
preferred embodiments taken in conjunction with the attached
drawings, in which:
FIG. 1 is a perspective diagrammatic illustration of a diamond grit
conditioner that is one prior art technique for reconditioning the
polishing surface of a polishing pad contaminated by its prior use
in a CMP process;
FIG. 2 is a perspective view of the apparatus of the invention;
FIG. 3 is an elevational view of the apparatus of FIG. 2;
FIG. 4 is a plan view of the invention taken perpendicular to a
plane defined by the width of the light beam;
FIG. 5 is a plan view similar to FIG. 4 showing a modification of
the invention wherein light beam scanning of the polishing pad is
provided by an oscillating or rotating mirror;
FIG. 6 is a plan view similar to FIG. 5 showing a further
modification of the invention wherein a second oscillating or
rotating mirror is used to expand coverage of the light beam
scanning of the pad; and
FIG. 7 is a sectional elevational view taken along line 7--7 of
FIG. 6.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
With reference to FIGS. 2-4, the polishing pad cleaning apparatus
and method of the present invention are illustrated. The cleaning
apparatus may be used in conjunction with a polishing apparatus
used to planarize a thin film layer formed over a substrate of a
semiconductor wafer. The thin film is typically an intermetal
dielectric layer formed between two metal layers of a semiconductor
device. The polisher has a platen 20 and a polishing pad 22
supported on the platen and attached thereto with an adhesive
compound (not shown). The platen 20 is carried for rotation in
either direction by a drive shaft 24.
In the present invention, the contaminants adhering as a coating
and/or embedded in the surface of the polishing pad 22 are removed
by a high intensity light beam 26 that is focused to irradiate an
irradiated area A having a length L and a width W. The length L
results from the horizontal dimension H of the incident light beam
and the width W results from the vertical dimension V of the
incident light beam, but the width W is significantly greater than
the vertical dimension V due to the projection caused by the
grazing angle G between the plane of the light beam and the plane
containing the planer surface of the polishing pad. As the grazing
angle G becomes less, the width W will increase.
The surface 28 of the pad 22 is preferably irradiated in the
presence of water or some other vaporizable liquid that may be
applied to the surface of the pad as a fan-shaped spray S issuing
from a nozzle 30 supplied with the liquid via a conduit 32
connected to a source of pressurized liquid (not shown).
Alternatively, the liquid may be supplied from a feed tube (not
shown) located above the center of the polishing pad 22, the
deposited liquid then being distributed centrifugally over the pad
surface by rotation of the platen 20. The vaporization of the water
or other liquid to steam or vapor by heat energy from the light
beam aids in releasing the coated and/or imbedded particles from
the pad material.
As may be seen best in FIGS. 3 and 4, the light beam 26 is a laser
beam or other collimated and/or focused light beam provided by an
optical system, generally designated 36, which includes a laser or
other light source 38. Issuing from light source 38 is a relatively
broad three-dimensional beam 40 that typically may have a
rectangular cross section. The beam 40 is reflected by a mirror 42
as a reflected beam 44 that may have substantially the same large
cross section as beam 40. Beam 44 passes through a spherical focus
lens 46 for providing a focused beam 48 having a focal point FP by
way of example. However, the beam 48 need not have a focus where
the lens 46 is a negative lens which provides input to the positive
cylindrical lens. The lens system may be Galilean, Newtonian,
Telecentric, or any other combination of lenses capable of
providing the high intensity light beam 26. The cylindrical lens 50
condenses the beam 48 along the y-axis perpendicular to its
directional axis X (FIG. 3) to provide the thin concentrated beam
26, which preferably has a thin rectangular cross section with a
small vertical dimension V relative to a much larger horizontal
dimension H.
The mirror 42 and the lenses 46 and 50 are mounted in a housing 52,
which is arranged for reciprocal sliding movement along a pair of
parallel guide rods 54 and 56 mounted on a frame, generally
designated 60. To provide the grazing angle G, the housing 52 is
tilted or inclined downward from back to front by making the rear
legs 81, 81 of frame 60 extend vertically to a higher level than
the front legs 71, 71 of frame 60. The housing 52 may be moved
laterally back and forth along the guide rods 54 and 56 in a
reciprocal motion represented by the arrow M by a threaded lead
screw 62, which is mounted for rotation on the frame 60 and driven
in either rotational direction by a reversible electric motor 64.
The motor 64 is secured by bolts to a cross member 51 of frame 60
and the distal end of lead screw 62 is journaled for rotation in a
bearing 53 bolted to a cross member 55 of frame 60. The guide rods
54 and 56 pass through respective journal blocks 66 and 68 carried
beneath the housing 52, and the lead screw 62 engages an internally
threaded drive block 70 carried beneath the housing 52.
The platen 20 preferably rotates, as represented by the arrow R, to
move the pad surface 28 rapidly through the irradiated area A.
Reciprocal movement of the housing 52 causes corresponding
reciprocal movement of the irradiated area A of light beam 26 in a
radial direction across the surface 28 of the polishing pad 22. The
range of such reciprocal radial movement may be relatively small
because the irradiated area A does not need to go past the
rotational axis of drive shaft 24. In addition, a central portion
of pad 22 would not be contaminated where polishing of the silicon
wafers takes place over only an annular outer portion of the
polishing pad surface 28.
The water nozzle 30 is carried by a frame member 34 that also may
be mounted for reciprocal motion for causing the water spray S to
reciprocate radially back and forth across the surface 28 of the
pad 22 as indicated by the arrow R' in FIG. 2. Alternatively,
nozzle 30 or a feed tube may be in a fixed position at or near the
center of the pad surface and centrifugal force relied upon to
spread the discharged water across the remainder of the pad
surface.
The optical assembly 36 is adjustable vertically by means of four
identical brackets 57, 59, 61 and 63, each releasably secured to
one of the angle iron frame members 71 and 81 by a pair of bolts
65, 65 that pass through clamping washers 67, 67 and slots 69, 69
in the corresponding adjustment bracket. The bolts 65, 65 then pass
through holes in the frame members and, on the inside thereof,
engage the threads of corresponding nuts 73, 73. Adjustments in the
tilt or inclination of the optical assembly 36 relative to the
plane of polishing pad 22, to thereby change the beam incidence
angle G, are provided by an arcuate slot 75 in each bracket,
through which passes a bolt 77 having a clamping washer 79 and
engaging a threaded bore (not shown) in the corresponding end of
guide rod 54 or 56.
Referring now to FIG. 5, there is shown a modified optical assembly
36' wherein an irradiated area A' is cause to reciprocate radially
across the pad surface 28 between locations L1 and L2 by
oscillating a mirror 72 with a galvanometer 74. The mirror 72 is
mounted for such oscillatory movement in a housing 76 on which the
galvanometer 74 is also mounted. Also mounted in casing 76 is a
lens 80 for focusing a reflected laser or collimated beam 82 onto
the pad surface 28. As a further alternative, the oscillating
mirror 72 may be replaced by a rotating mirrored polygon (not
shown) driven in rotation by an electric motor mounted in place of
galvanometer 74.
Since the light source and mounting frame may be the same as those
of the first embodiment described above, the same numerals have
been used in FIG. 5 to designate the same elements of these
components. However, the casing 76 is mounted in a fixed lateral
position on the frame 60 relative to the rotational axis of the
platen 20 by fixing the casing 76 to front and rear lateral members
79 and 83. Lateral members 79 and 83 are in turn adjustably secured
to frame 60 by brackets 57, 59, 61 and 63 for vertical and tilting
adjustments of casing 76 relative to the pad surface 28 in the same
manner as for casing 52.
In FIG. 6 and 7, there is shown a further modification of the
invention wherein a greater pad area or a larger polishing pad
surface 28' is covered by the use of two optical systems, generally
designated 36' and 90, respectively. As the optical system 36' and
the mounting frame 60 may be the same as those of the FIG. 5
embodiment described above, the same numerals have been used in
FIG. 6 to designate the same elements of these components. As is
housing 76, the second housing 94 is similarly fixed to the frame
60. Also similar to the FIG. 5 embodiment, the second mirror 92 is
oscillated by a galvanometer 98 to provide an oscillating light
beam 120. However, only one of the two light beams 82 and 120 is
provided at a time because when the mirror 92 is reciprocated from
a retracted position P1 to an extended position P2, the mirror 92
intercepts the source light beam 40 and thereby prevents it from
reaching the more distant mirror 72.
To move the mirror 92 from its extended position P2 to its
retracted position P1, the optical assembly 90 includes a piston
108 arranged for reciprocating movement in a fluid cylinder 110 and
connected to a shuttle 100 by a piston rod 106. As may be seen best
in FIG. 7, the shuttle 100 is mounted for reciprocal movement in a
pair of guide tracks 102 and 104 and carries the mirror 92 and its
galvanometer 98.
The shuttle 100 and the mirror elements carried thereon are moved
to the retracted position P1 when the cylinder 110 is pressurized
by fluid entering the cylinder 110 through a supply line 114, which
causes the piston 108 to compress a return spring 112. When the
mirror 92 is in its retracted position P1, the source beam 40 is
allowed to pass through the housing 94 and into the housing 76,
where it is reflected by the oscillating mirror 72 and relayed
through the optical lens 80 to the pad surface 28' as the focused
light beam 82. The light beam 82 is thereby caused to oscillate
over a sufficient radial distance to cover an outer section of the
surface 28' of polishing pad 22'. When the cylinder 110 is not
pressurized to actuate the piston 108, the spring 112 returns the
mirror 92 to its extended position P2, in which position this
mirror relays the source beam 40 through the optical lens 107 to
the polishing pad surface 28' as the focused light beam 120. The
oscillation of beam 120 covers an inner section of the polishing
pad surface 28'. The irradiated area A1 by beam 82 preferably
overlaps the irradiated area A2 by beam 120 to provide an overlap
area OL, which is at the inner limit of the oscillation of beam 82
and the outer limit of the oscillation of beam 120.
The dual optical system arrangement of the FIG. 6 allows a piston
and shuttle to take the place of the lead screw and motor assembly
used in the embodiment of FIGS. 2-4. Such a dual optical system may
be significantly less expensive compared to the lead screw and
motor assembly, and may also require significantly less
maintenance. A similar dual optical system may employ two fixed
housings as in FIG. 6 each with a fixed mirror as in FIG. 4, but in
this alternative, the housings would be canted relatively to each
other so that the projected beams would have an overlap similar to
the overlap OL in FIG. 6. It more area is required to be covered in
either of the foregoing alternatives, such as for the purpose of
extending coverage over the entire diameter of the pad, these
arrangements may be duplicated by additional housings each with a
retractable mirror similar to mirror 92.
The above two alternatives for moving the mirror relative to the
casing are known as Pre-Objective Scanners. When the mirrored
surface changes its angle relative the incident beam 40, the
mirrored surface directs the beam at a different angle and position
into the lens 80. The lens 80 then focuses the reflected beam 82 at
different locations between the limit locations represented by
locations L1 and L2 so as to fully cover the scanned area C on the
surface 28 of pad 22. The advantages of Pre-Objective Optical
Systems as shown in FIG. 5 are their high scan speed and their
ability to have a focused image at the scanned surface. However,
for the scanning beam to stay in focus, the lens 80 must be large
and the optics relatively complex and expensive.
A less expensive alternative is a telecentric optical system that
allows the center of the scanning beam of light to impinge on a
work surface orthogonally throughout its scan. Accordingly, while a
Pre-Objective Optical System has been illustrated in FIG. 5, less
expensive telecentric scanning systems may be available, such as
those shown and described in U.S. Pat. No. 5,620,618 in the name of
Freedenberg, et al., and assigned to the same assignee as the
present disclosure. The entire contents of said U.S. Pat. No.
5,620,618 are incorporated herein by reference.
The preferred light source is an excimer laser having output energy
preferably in the range of 20 to 2,000 mj and a frequency
preferably in the range of 20 to 1,000 hz. The rotational speed at
the edge of the platen during reconditioning is preferably in the
range of 10 to 500 millimeters per second at an incidence angle G
in the range of 0.degree. to 90.degree., preferably 5.degree. to
45.degree., more preferably 10.degree. to 30.degree.. The size of
the irradiated area A preferably has a length in the range of 5 to
25 cm, more preferably 10 to 21 cm and most preferably about 21 cm;
and a width W in the range of 0.1 to 0.8 cm, preferably 0.2 to 0.6
cm, more preferably about 0.3 cm.
A preferred laser light source is a XeCl excimer providing a
minimum output energy of 100 mj at a wave length of 308 nanometers
and a frequency of 50 to 500 hz. A more preferred laser light
source is a KrF excimer providing a minimum output energy of 50 mj
at a wavelength of 248 nanometers and a frequency of 50 to 200 hz.
A most preferred laser light source is an ArF excimer providing a
minimum output energy of 20 mj at a wavelength of 193 nanometers
and a minimum frequency of 50 hz.
As one specific example using an ArF excimer, the output energy
density is 50 mj/cm.sup.2 at a frequency of 100 hz with an average
linear scanning speed of 10 mm per second, an incidence angle of
15.degree., and an irradiated area A measuring about 10 cm in
length and 0.3 cm in width. Where the light source is a laser, the
preferred energy density in the irradiated area A at the work
surface of the polishing pad is preferably in the range of 800 to
3000 mj/cm.sup.2, more preferably 800 to 2000 mj/cm.sup.2, and most
preferably 800 to 1200 mj/cm.sup.2. A power density at the work
surface of 1000 mj/cm.sup.2 may be achieved by an ArF excimer
operating at a wavelength of 193 nanometers and delivering at the
work surface a peak power of 100 megawatts per cm.sup.2 per pulse
of 10 nanosecond duration, and at a frequency of 100 hz (100 pulses
per second). As will be understood by persons skilled in the art,
the parameters of this example are scalable to decrease the pad
reconditioning time, such as by increasing the energy density and
increasing the platen rotational speed.
Instead of a laser, the light source may be a mercury arc lamp with
a collimating optical system for providing collimated ultraviolet
light, such as ultraviolet light with a wavelength in the range of
365 to 410 nanometers. The output power of the arc lamp may be in
the range of 300 to 600 watts per inch. The radiation density
provided in the irradiated area at the work surface may be
significantly less than that provided by a laser since mercury arc
lamp irradiation is based on continuous average power rather than
short pulses of high peak power.
Persons skilled in the art, upon learning of the present
disclosure, will recognize that various modifications to the
elements and steps of the invention are possible without
significantly effecting their functions. For example, the light
sources and optical arrangements described above may be varied
widely in accordance with technology currently available for
providing concentrated beams of light. Similarly, means other than
those disclosed may be used to cause the light beam to scan and the
liquid spray to move across the contaminated surface of the
polishing pad. In addition, the polishing pad itself may be
reciprocated laterally past a fixed light beam to produce the same
relative movement between the light beam and the pad as provided by
a scanning light source. Accordingly, while the preferred
embodiments have been shown and described in detail by way of
example, further modifications and embodiments are possible without
departing from the scope of the invention as defined by the claims
set forth below.
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